The sun is our principle source of energy, manifesting itself mainly in the form of light and heat, issuing electromagnetic radiation that travels through space in the form of waves and particles. The waves are described by their frequency (ν) or their wavelength (λ). The following table shows the solar electromagnetic radiation spectrum:
TABLE 1Solar Radiation SpectrumSolar RadiationWavelengthCosmic Rays0.005 ÅGamma Rays0.005-1.4 ÅX-Rays0.1-100 ÅUltraviolet C (UVC)200 nm-280 nmUltraviolet B (UCB)280-320 nmUltraviolet A I (UVA I)320-340 nmUltraviolet A II (UVA II)340-400 nmVisible Light400-740 nmNear-Infrared740 nm-1,500 nmMid-Infrared1,500-5,600 nmFar-Infrared5,600-10,500 nmMicrowaves and Radio Waves>10,600 nm
Not all solar radiations reach earth since part of these are reflected, absorbed or dispersed because the Earth is protected by atmospheric gas layers that filter and attenuate the solar radiations. Only radiation from 290 to 1,800 nm (UVB, UVA (I and II), visible and near infrared) reach the surface of the earth. Of this range, UVB and UVA (I and II) are the ones that reach the surface the least as they are filtered by the ozone layer, but they are the ones that most affect the biosphere, including humans. On the other hand, the harmful effects of solar radiation is attracting more and more attention, particularly because of the of the ozone layer depletion phenomenon, which reduces the filtering effect on solar radiation (see Ozone Depletion and Human Health Effects, by M. J. Molina and Luisa T. Milina, Environmental Medicine; L. Möller Ed. Sep. 24, 2002 ENVIMED).
Overexposure to UVB radiation produces harmful effects on the skin in the short-term, producing erythemas, which is the well known inflammation process with reddening of the skin. Overexposure to UVA, on the other hand, produces harmful effects in the longer term. We may observe a graph that has been made public showing the solar spectrum reaching earth in terms of wavelength, the erythemal action spectrum, and the relationship between them in FIG. 1. This graph shows what has been indicated above, that is, that erythema is produced mainly by the incidence of UVB radiation on the skin, whereas UVA I and II (320-400 nm) does not produce such a skin reaction. UVA I and II, however, produce other long-term effects that are much more harmful, such as photoaging and photocarcinogenesis (see Photocarcinogenesis: UVA vs UVB Radiation, by Fr. R. de Grujil, Skin Pharmacol. Appl. Physiol. 2002; 15:316-320).
The principal differences between the harmful effects of UVB and UVA radiation on the skin are presented in the following Table:
TABLE 2Comparative Effects of UVB and UVA RadiationSkinPhotodynamicRadiationEnergyPenetrationDamageActionUVB+++LittleDirect—UVA+SubstantialDirect and IndirectSubstantial
The skin has chromophores capable of absorbing UVA radiation, primarily melanin and urocanic acid, in addition to nucleic acids and protein aromatic residues.
Because of its greater skin penetration, UVA radiation has a longer term effect on skin degeneration in view of its higher photodynamic action. The photodynamic action produced by UVA radiation is a result of the reaction of the UVA (I and II) energy (hν) with photosensitizers in the skin or the environment in the presence of oxygen in the air. This produces reactive oxygen species (ROS), which are known as free radicals and singlet oxygen (see “Sunscreen enhancement of UV-induced reactive oxygen species in the skin”, Ferry M. Hanson et al., Free Radical Biology & Medicine 41 (2006) 1205-1212). Free radicals are, by definition, reactive chemical species that contain non-paired electrons in their respective orbitals, and that may be neutral, negatively or positively charged. They are highly unstable and therefore tend to react, altering the cellular components. They are therefore associated with photoaging, melanoma and skin cancer, among other diseases (see “Free Radicals in Cutaneous Biology”, J. Invest. Dermatol. 102:671-675, 1994 and “Cutaneous Photodamage, Oxidative Stress and Topical Antioxidant Protection”, J. Am. Acad. Dermatol. 2003; 48:1-19, Tedesco A C et al, 1997).
There are many studies on chemical and physical filters focused on avoiding these harmful effects on the skin, such as “Photoprotection” by P. Kullavanijaya and H. W. Lim, J. Am. Acad. Dermatol. 2005; 52:937-58 and “Ultraviolet Radiation Screening Compounds”, Biol. Rev. (1999), 74, pages 311-345, whose purpose is topical photoprotection by means of substances that absorb and filter UVB and UVA radiation (chemical filters); that inactivate or destroy the reactive oxygen species (free radicals and singlet oxygen) that are produced in the skin by means of antioxidants; or that reflect the radiation by dispersion with physical filters such as TiO2 or ZnO. In fact, it has been make known that there are vegetable extracts with antioxidant properties able to offset the oxidative effects induced by TiO2 (see “Plypodium Leucotomos Extract Inhibits Trans-Urocanic Acid Photoisomerization and Photodecomposition”, Journal of Photochemistry and Photobiology B: Biology 82 (2006) 173-179).
On the other hand, the effect of UVB and UVA ultraviolet radiation also affects plants (see Journal of Photochemistry and Photobiology B: Biology, Volume 76, Issues 1-3, Oct. 25, 2004, Pages 61-68), with plants having their own defense mechanisms, as not only men naturally generate photoprotective substances, such as melanin. Plants also generate their own defenses, such as Deschampsia Antarctica, which grows under very low temperatures with spells of very high solar radiation. This has lead it to develop effective defense mechanisms to cope with these extreme conditions. It is able to dissipate the reactive oxygen species (ROS) by developing an elevated antioxidant capacity, along with a large capacity to process the excess UV radiation non-radiatively as heat in small quantities. This plant is peculiar in that it grows in the Antarctic Continent and tolerates the extreme conditions in its habitat without problems. It is able to stay green throughout the year, even under ice and snow during the Antarctic winter, being one of the few plants able to tolerate such extreme climatic conditions. (see “The Role of Photochemical Quenching and Antioxidants in Photoprotection of Deschampsia Antarctica”, in Functional Plant Biology, 2004, 31, 731-741).
There is today a large necessity to obtain products for durable skin protection against the adverse effects produced by UVA and UVB radiation. Although it is known that an antioxidant neutralizes the photodynamic action produced by reactive oxygen species (ROS), it is necessary to detect which antioxidants are adequate and to verify their effects, since not all are beneficial, as they can produce chain propagation and other even worse consequences. Various biological factors, such as skin tropism, also have to be considered.